Amazon Rainforest Greens Up in the Dry Season

The Amazon rainforest puts on its biggest growth spurt during the dry season, according to new research. The finding surprised the researchers.

“Most of the vegetation around the world follows a general pattern in which plants get green and lush during the rainy season, and then during the dry season, leaves fall because there’s not enough water in the soil to support plant growth,” said lead researcher Alfredo R. Huete of The University of Arizona in Tucson.

“What we found for a large section of the Amazon is the opposite. As soon as the rains stop and you start to enter a dry period, the Amazon becomes alive. New leaves spring out, there’s a flush of green growth and the greening continues as the dry season progresses.” The paper by Huete and eight colleagues in the United States and Brazil is scheduled for publication on 22 March in Geophysical Research Letters.

This finding holds true only for the undisturbed portion of the rainforest. Areas where the primary forest has been converted to other uses or disturbed “brown down” in the dry season, said Huete, a professor of soil, water and environmental science.

Huete suggests the deep roots of trees in the undisturbed forest can reach water even in the dry season, allowing the trees to flourish during the sunnier, drier part of the year. In contrast, plants in areas that have been logged or converted to other uses cannot reach deep water in the dry season and therefore either go dormant or die.

The researchers say that figuring out the metabolism of the Amazon, the largest old-growth rainforest on the planet, is crucial for understanding how rainforests and other tropical environments function and how deforestation affects biodiversity and sustainable land use in the tropics. It will also help scientists better understand the global carbon cycle, which includes the natural sequestration and release of carbon dioxide, a major greenhouse gas.

The finding that converted forests grow differently from undisturbed forests has implications for understanding the effects of fires in the tropics, including the fires that sometimes rage in tropical areas during El Nino years, which bring drought to many tropical areas, including the Amazon.

The research team analyzed five years of satellite images from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument mounted on NASA’s Terra satellite and by cross-checking with information from sites on the ground. To determine when the Amazon rainforest is growing, Huete’s lab used a new measure, called Enhanced Vegetation Index (EVI), for detecting greenness in MODIS images of very highly vegetated rainforests. Growing plants generate more chlorophyll and therefore look greener.

“We can look at this increase in greenness as a measure of Amazon health, because in the disturbed areas we don’t see the greenness increase during the dry season,” Huete said. “A lot of people are interested in the rainforest because of the humongous amount of carbon it stores. A very slight change in the forest’s activity will make a tremendous change in the global carbon cycle.”

“With the satellite, we can say the whole Amazon basin is doing something,” Huete said. The team’s next step, Huete said, is to see if other tropical rainforests behave the same way and how the rainforests behave in El Nino years. He added, “We also want to look harder at the transition zones at the edge of the rainforest to see whether different kinds of disturbance cause different growth patterns.”

The research was funded by NASA and is part of the Brazilian-led Large Scale Biosphere-Atmosphere Experiment in Amazônia (LBA).